Testing Your Program CS 1111 Introduction to Programming

Testing Your Program CS 1111 Introduction to Programming Spring 2019 CS 1111 – University of Virginia © Praphamontripong

Testing and Debugging • Testing = process of finding input values to check against a software Test case consists of test input values and expected results Test values (inputs) Program Actual results vs. Expected results • Debugging = process of finding a defect given a failure CS 1111 – University of Virginia © Praphamontripong 2

Why do We Test Software? • Goal of testing • Not to prove correctness, but to increase our confidence in correctness • Improve quality • Reduce overall software development cost (budget, time, and effort) • Preserve customer satisfaction • Get good grades in CS 1110/1111 and any programming courses • What fact does each test try to verify? • Know what to check and whether the program handles that properly • Benefits • You are not biased that your code works • You will better understand what you need to build • You will get insights on how to built it CS 1111 – University of Virginia © Praphamontripong 3

An Example in Python # Return index of the first occurrence of a letter in string, # Otherwise, return -1 def get_index_of(string, letter): index = -1 for i in range(1, len(string)): if string[i] == letter: index = i return index Defect: Should start at 0, not 1 Test 1: not reveal a problem (defect) actual output = expected output # Test 1: inputs “python”, “z” print(get_index_of("python", "z")) # expected: -1, actual: -1 # Test 2: inputs “python”, “z” print(get_index_of("python", "p")) # expected: 0, actual: -1 Test 2: reveal a problem (defect) actual output ≠ expected output For simplicity, this example assumes a function accept a letter of size 1 CS 1111 – University of Virginia © Praphamontripong 4

Testing: Choosing Test Inputs # Return index of the first occurrence of a letter in string, # Otherwise, return -1 def get_index_of(string, letter): index = -1 for i in range(1, len(string)): if string[i] == letter: index = i return index Focus on input values Focus on program functionality 1. Identify inputs • string, letter 2. What input values can be • string is empty or not • letter is empty or not • length of string (0, 1, 2, >2) CS 1111 – University of Virginia 2. What affect program’s functionality • number of occurrence of letter in string • letter occurs first in string • letter occurs last in string © Praphamontripong 5

Testing: Comparing Results • Given the test input values, compare the actual results with the expected results • If the actual results == expected result, the program passes the test • Otherwise, the program fails the test • The test input valules reflect the characteristics of the input parameters • The characteristics of the inputs signify the kinds of defects in program • The kinds of defects tell what to fix CS 1111 – University of Virginia © Praphamontripong 6

Debugging # Return index of the first occurrence of a letter in string, # Otherwise, return -1 def get_index_of(string, letter): Fix this index = -1 for i in range(1, len(string)): # check if we get the right character print(i, string[i]) if string[i] == letter: index = I # check if the if-code-block is executed print(string[i], “=”, letter, “i=”, i) Run Test 2 1 1 y 2 t 3 h 4 o 5 n -1 return index # Test 1: inputs “python”, “z” print(get_index_of("python", "z")) # expected: -1, actual: -1 # Test 2: inputs “python”, “z” print(get_index_of("python", "p")) # expected: 0, actual: -1 CS 1111 – University of Virginia © Praphamontripong 7

Summary Debugging Testing • Choose test values; check if the program fails (i. e. , there is a problem or “defect” in the code, which causes the program to fail) • Each test value serves a single purpose (or check for a certain aspect) • Check • Normal cases • Corner, edge, boundary cases • Exceptional cases CS 1111 – University of Virginia • The program failed; find where to fix • Use “print” statement to ensure changes to the program variables are correct • Verify forward: print from start, move forward until incorrect values are detected • Verify backward: print from the end, move backward until incorrect values are detected © Praphamontripong 8